Deliantra is a free software, cooperative multiplayer RPG and adventure game, much similar in style (and setting) to nethack and moria (It derives from Crossfire project), but is fully graphical and runs in realtime. It is set in a classical medieval environment with ubiquitious weapons and magic.
Large world to explore (multiple continents, over 4000 manually designed maps, hundreds of monsters and countless variations of generated content) and complex (16 unique races, 17 classes, 40 skills and hundreds of spells to choose from and combine with each other) is what makes the game special.
The gameplay is open and sets very few limits on character development. And of course a lot of fun is to be had by playing cooperatively with others exploring the same world.
(Note: There are also Mac OS X and Windows binaries available.)